In systems for wireless transmission of information using electromagnetic signals, for example radar and cellular telephony or some other telecommunication area, there is a strong need for efficient antennas, both single antennas and group or array antennas. For different applications, different types of antennas with different properties are desired. For many applications, broadband properties are desired.
When an antenna element is used in an array, i.e. when a number of antenna elements are placed in a horizontal row or a vertical column, the antenna element may be fed with varying phase, which results in that the main lobe of the array antenna radiation pattern may be directed in different directions along the array. A two-dimensional array may also be used, where a number of antenna elements are placed in horizontal rows and vertical columns. The elements may then be fed with varying phase along both the horizontal rows and the vertical columns allowing the main lobe of the array antenna radiation pattern to be directed in different horizontal and vertical directions along the array. These “steerable” arrays are also called phased arrays.
Antenna elements may also be arranged in orthogonally arranged pairs, radiating in orthogonal directions. These antennas are called dual polarized antennas. An array antenna may thus be dual polarized if it consists of an equal amount of orthogonally arranged pairs of antenna elements. One reason for using a dual polarized antenna is that so-called polarisation diversity is desired. Polarisation diversity is for example desired when there is a risk that the antenna signal is reflected in such a way that the main signal and the reflected signal have opposite phases at the point of reception, causing the signal to fade out. If two polarizations are used, the risk of fading decreases as both polarizations would have to fade at the same time.
One kind of non-resonant antenna element which typically is used when a wide broadband performance is desired, i.e. when a performance over a wide frequency span is desired, is the so-called notch antenna, which is a kind of a so-called end-fire element. Also, when used in an array antenna, the use of notch antenna elements allows the array antenna to be directed to scan wide angles. Especially, the use of a tapered notch antenna element is preferred, which basically comprises a slot in a metal layer, which slot widens as it approaches an edge of the metal layer.
One special kind of a tapered notch antenna element is the so-called Vivaldi notch antenna element, which may be used alone or in an array.
A typical tapered notch antenna element may be formed on a first copper-clad substrate, for example a PTFE-based substrate, where the copper on one side, the feeding side, has been etched away but for a single feeding microstrip line. On the other side of the substrate, a slot is formed in the copper, which slot starts to widen as it approaches an edge of the substrate, forming a tapered slot. The tapering is typically represented by an exponential form. The microstrip feeding line passes the slot on the other side of the substrate in such a way that the longitudinal extension of the microstrip feeding line is essentially perpendicular to the longitudinal extension of the slot. The microstrip feeding line passes the slot approximately with the length λg/4, i.e. one quarter of a wavelength in the material, a so called guide wavelength, if the feeding line is open-ended. The open-ended feeding line transforms to a short-circuited feeding line under the slot due to the λg/4 length. The microstrip feeding line then couples energy to the slot, as the electromagnetic field of the microstrip feeding line is interrupted by the slot.
This design is, however, asymmetrical when looking towards the edge of the laminate where the tapered slot emerges, as there is a feeding line on one side of the laminate and a tapered slot structure on the other side. This asymmetry may result in cross-polarization at the antenna radiation pattern. One way to come to terms with this asymmetry is to mount a second laminate, without copper on one side and with an essentially identical tapered slot structure on the other side, to the first laminate in such a way that the side without copper on the second laminate faces the side with the microstrip feeding line on the first substrate. In this way the feeding line is squeezed between the two laminates, forming a stripline feeding line, with essentially identical tapered slots etched out of the copper cladding on the outer sides, forming a dual-sided notch antenna.
The basic configuration of a tapered slot antenna element of the Vivaldi type is described in the technical article “Wideband Vivaldi arrays for large aperture antennas” by Daniel H. Shaubert and Tan-Huat Chio. There the λg/4 length is made as a so-called radial stub in order to achieve a larger bandwidth. The other end of the slot, opposite to the tapered part of the slot, is ended with a circular part without copper, forming a two-dimensional cavity which results in an open-ended slot line close to the feeding point. The article also describes how array antennas may be formed using a Vivaldi antenna element. A problem with this symmetrical Vivaldi antenna element design is that so-called parallel plate modes appear in the substrate material, i.e. undesired propagation of electromagnetic radiation. In order to suppress these parallel plate modes, metallic posts, vias, have to connect the copper on the outer sides of the laminates, surrounding the tapered slot structure.
This dual sided tapered slot antenna with vias for mode suppression ends up in a rather complicated substrate configuration, especially in an array configuration. The use of substrates renders dielectric losses and also makes the final antenna quite heavy. The use of substrate materials is also disadvantageous when an antenna is meant to be used for space applications, i.e. in a satellite, as electrostatic build-ups in the plastic material may result in discharges that could be fatal for adjacent electronic circuits. The common PTFE substrates are also relatively expensive.
U.S. Pat. No. 5,142,255 describes co-planar waveguide filters etched on a substrate, which filters may be combined with a notch antenna which is fed by active components. This is however a quite narrow-banded structure, as the co-planar waveguide filters are resonant for certain narrow frequency bands. The active components may also affect the bandwidth of the structure.
Neither of the documents above disclose how a broadband, symmetrical tapered slot antenna element that does not have to be supported by a substrate may be devised.